The invention relates to an adjusting method such as color matching adjustment for an image forming apparatus by electrophotographic system and an image forming apparatus and, more particularly, to an image adjusting method and image forming apparatus for automatically adjusting color misregistration of multicolor image occurring at the time of forming a multicolor image by overlaying color component images formed on an image carrier (image forming means; photosensitive drum) or transfer carrier (transfer medium: transfer belt, paper).
An image forming apparatus such as a digital color copier or digital color printer is designed to separate inputted data into a plurality of color components and process the image, and form a multicolor image by overlaying images for each color component. At the time of forming a multicolor image, when images of each color component are not overlaid correctly, color misregistration occurs in the formed multicolor image, and the image quality may be lowered. In particular, to enhance the forming speed of multicolor image, in an image forming apparatus having an image forming unit for each color component, an image of each color component is formed in each image forming unit, and formed images of color components are overlaid sequentially, so that a multicolor image may be formed. In such image forming apparatus, a transfer position of image of each color component is likely to be deviated, and color misregistration of multicolor image is a serious problem.
Accordingly, in the image forming apparatus, in order to overlay the images of color components at high precision, by executing a color matching adjustment (registration adjustment) for adjusting the color misregistration of multicolor image, it is intended to form a favorable multicolor image free from color misregistration. In color matching adjustment, usually, the misregistration of an image forming position of other color component with respect to the image forming position of a reference color component is detected by using an optical detector. The adjusting amount is determined on the basis of the result of this detection and, according to this adjusting amount, the timing for forming the image of each color component is adjusted so as to match the transfer position of image of each color component. In order to determine the adjusting amount, generally, the image of each color component is transferred at the same timing, and the distance between transfer positions of color components is detected or the density of the multicolor image overlaying the color components is detected.
For example, in the image forming apparatus disclosed in Japanese Patent Application Laid-Open No. 10-213940(1998), the distance between transfer positions of images of color components is detected, and it is adjusted according to the detected misregistration amount of transfer position. That is, the distance between the image formed by reference color component and the image formed by other color component is detected by a detector, and the misregistration amount of the transfer position of images of color components is determined on the basis of the detected distance, and thereby the color misregistration is adjusted.
Japanese Patent Application Laid-Open No. 2000-81744 discloses an image forming apparatus designed to adjust the color misregistration by measuring the density of multicolor image by overlaying images of color components and adjusting so that the measured density may coincide with the density in the correctly overlaid state of images of color components. In this image forming apparatus, to enhance the precision of adjustment, images of color components are formed by repeating a plurality of same images. As the same images, a plurality of line images are formed, and the density of multicolor line images is detected by a detector, and the overlaid state of the line images of color components is obtained. When the density of the multicolor line images detected by the detector is within the specified density range, it is determined that the line images of color components are overlaid correctly, and the color matching adjustment is executed so that the image may be formed in this overlaid state.
When forming images, measuring the position and density of formed images, detecting the relative position of the color image requiring adjustment on the basis of a reference color image, and executing the color matching adjustment in this manner, the result of detection depends very much on the quality of the formed images (size, density, edge state). By contrast, Japanese Patent Application Laid-Open No. 10-260567(1998) discloses a technique of executing the color matching adjustment by forming an image for density control, adjusting the image forming condition when its density does not reach the specified level, and matching the color by the image for color matching adjustment.
However, in the image forming apparatus of tandem system constituted by a plurality of image forming units for forming images of color components arranged in line, the distance from the image forming units to the detector for detecting the image for adjustment is very long, and once an image for color matching is formed before checking the density as the detector detects the image for density control, wasteful image forming is executed when the density does not reach the specified level.
Besides, the image forming apparatus disclosed in Japanese Patent Application Laid-Open No. 2000-81744 is designed to execute the image color matching adjustment by the image for density detection, and it consumes much developer.